The responses of several adrenal steroids to adrenocorticotropin (ACTH) were investigated in a group of healthy young men. The protocol consisted of overnight adrenal suppression with dexamethasone followed by 1 hr infusions of ACTH in increasing (4X) doses ranging from 30 - 30,720 ng/1.5M2 followed by 0.25 mg ACTH/1.5M2. Whereas cortisol, androstenedione and androstenediol concentrations tended to plateau at the higher ACTH infusion rates, those of dehydroepiandrosterone (DHEA) and 17-hydroxyprogesterone (17-OH P) did not. The cortisol concentration achieved after the highest dose of ACTH was over 50 fold higher than that at baseline. DHEA levels rose to values about 13 times baseline whereas the other steroids increased to lesser extents (3-5 fold). The ACTH infusion rate required to significantly increase each steroid over the baseline level was extrapolated from dose response curves and was considered to be an index of the sensitivity of each steroid to ACTH. Cortisol was the most sensitive, rising significantly at approximately 35 ng ACTH/1.5M2, followed by androstenediol, DHEA, androstenedione and 17- OH P.